\sum_k=1^50 \left( \frac12k - \frac1k+1 + \frac12(k+2) \right) = \frac12 \sum_k=1^50 \frac1k - \sum_k=1^50 \frac1k+1 + \frac12 \sum_k=1^50 \frac1k+2. - Portal da Acústica